Extinction Of Local Populations Research Articles

Fine-scale biostratigraphy within the Stimulograptus sedgwickii Zone (Silurian: Llandovery) at Dob’s Linn, Southern Uplands

Synopsis A 1.89-m section excavated from the Stimulograptus sedgwickii Biozone (Silurian: Llandovery, Aeronian) at Dob’s Linn, Southern Uplands, Scotland, representing some 0.2 Ma of deposition, shows pronounced, quantitatively established patterns of graptolite abundance and diversity. The most striking signature comprises several marked peaks in abundance of the distinctive taxon Neolagarograptus tenuis (Portlock) within this interval. Such ‘bloom events’, if recognized further afield, may provide very fine-scale stratigraphic resolution, seemingly akin to the ammonite biohorizons of the Jurassic. Taxonomic richness as a whole decreases upwards and appears associated with bentonite layers, suggesting that volcanic ash falls might have led to local extinctions of graptolite populations with little or no subsequent recovery. Repetition of the analysis on a duplicate section along-strike at Dob’s Linn showed comparable patterns, demonstrating the robustness of the methodology.

Invasion success despite reduction of genetic diversity in the European populations of eastern mosquitofish (Gambusia holbrooki)

The introduction of a few individuals to new, isolated habitats (founder effect) is expected to reduce the genetic variability of a population. At the beginning of the last century a few eastern mosquitofish (Gambusia holbrooki) individuals were introduced to Southern Europe from North America to control malaria‐carrying mosquitoes. We studied the effect of this severe bottleneck on genetic variability in four populations of eastern mosquitofish introduced to Spain and Italy in the early 1900s and compared them to a native population in North America. Using amplified polymorphic DNA–polymerase chain reaction (RADP–PCR) we found a strong reduction of genetic diversity, in terms of both number of polymorphic loci and heterozygosity, in all four European populations. Despite this great reduction in genetic diversity, eastern mosquitofish successfully invaded European internal waters causing eutrophication problems and threatening the extinction of local populations of native fish.

Antagonisms, mutualisms and commensalisms affect outbreak dynamics of the southern pine beetle

Feedback from community interactions involving mutualisms are a rarely explored mechanism for generating complex population dynamics. We examined the effects of two linked mutualisms on the population dynamics of a beetle that exhibits outbreak dynamics. One mutualism involves an obligate association between the bark beetle, Dendroctonus frontalis and two mycangial fungi. The second mutualism involves Tarsonemus mites that are phoretic on D. frontalis ("commensal"), and a blue-staining fungus, Ophiostoma minus. The presence of O. minus reduces beetle larval survival ("antagonistic") by outcompeting beetle-mutualistic fungi within trees yet supports mite populations by acting as a nutritional mutualist. These linked interactions potentially create an interaction system with the form of an endogenous negative feedback loop. We address four hypotheses: (1) Direct negative feedback: Beetles directly increase the abundance of O. minus, which reduces per capita reproduction of beetles. (2) Indirect negative feedback: Beetles indirectly increase mite abundance, which increases O. minus, which decreases beetle reproduction. (3) The effect of O. minus on beetles depends on mites, but mite abundance is independent of beetle abundance. (4) The effect of O. minus on beetles is independent of beetle and mite abundance. High Tarsonemus and O. minus abundances were strongly correlated with the decline and eventual local extinction of beetle populations. Manipulation experiments revealed strong negative effects of O. minus on beetles, but falsified the hypothesis that horizontal transmission of O. minus generates negative feedback. Surveys of beetle populations revealed that reproductive rates of Tarsonemus, O. minus, and beetles covaried in a manner consistent with strong indirect interactions between organisms. Co-occurrence of mutualisms embedded within a community may have stabilizing effects if both mutualisms limit each other. However, delays and/or non-linearities in the interaction systems may result in large population fluctuations.

INVESTIGATING THE POPULATION-LEVEL EFFECTS OF CHYTRIDIOMYCOSIS: AN EMERGING INFECTIOUS DISEASE OF AMPHIBIANS

Chytridiomycosis is an emerging infectious disease that has recently been reported in amphibian populations throughout the world. It has been associated with many cases of population declines and extinctions. In some areas of the Sierra Nevada of California the disease appears to be the causal factor in the rapid extinction of local populations of the mountain yellow-legged frog, Rana muscosa, within a few years of the first detection of the disease. In other areas, however, R. muscosa populations appear to persist for many years, despite high levels of infection in tadpoles. Here we present simple models of the dynamics of the disease within an individual lake and ask whether our current understanding of the disease is consistent with the field survey observations of: (a) extinction due to the disease over a wide range of host population sizes, and (b) persistence of frog populations with the disease at some sites. Despite our laboratory observation of chytridiomycosis being invariably lethal to postmetamorphic frogs, the observed long-term persistence of infected frog populations can only be explained if at least some infected adult frogs survive and reproduce.

Genetic consequence of restricted habitat and population decline in endangered Isoetes sinensis (Isoetaceae).

Isoetes sinensis (Isoeteaceae) is a critically endangered aquatic quillwort in eastern China. Rapid decline of extant population size and local population extinction have occurred in recent years and have raised great concerns among conservationists. Amplified fragment length polymorphisms (AFLPs) were used to investigate the genetic variation and population structure of seven extant populations of the species. Eight primer combinations produced a total of 343 unambiguous bands of which 210 (61.2 %) were polymorphic. Isoetes sinensis exhibited a high level of intra-population genetic diversity (H(E) = 0.118; hs = 0.147; I = 0.192; P = 35.2 %). The genetic variation within each of the populations was not positively correlated with their size, suggesting recent population decline, which is well in accordance with field data of demographic surveys. Moreover, a high degree of genetic differentiation (F(ST) = 0.535; G(ST) = 0.608; theta(B) = 0.607) was detected among populations and no correlation was found between geographical and genetic distance, suggesting that populations were in disequilibrium of migration-drift. Genetic drift played a more important role than gene flow in the current population genetic structure of I. sinensis because migration of I. sinensis is predominantly water-mediated and habitat range was highly influenced by environment changes. Genetic information obtained in the present study provides useful baseline data for formulating conservation strategies. Conservation management, including both reinforcement for in situ populations and ex situ conservation programmes should be carefully designed to avoid the potential risk of outbreeding depression by admixture of individuals from different regions. However, translocation within the same regional population should be considered as a measure of genetic enhancement to rehabilitate local populations. An ex situ conservation strategy for conserving all extant populations to maximize genomic representation of the species is also recommended.

Is the population turnover of patchy‐distributed annuals determined by dormancy dynamics or dispersal processes?

In species with fragmented distribution, regional turnover dynamics is given by the processes of local population extinction and patch (re)colonization by migrants spreading from neighboring occupied patches. In plants with dormant stages (e.g. seeds) and limited dispersal capacity, regional dynamics based on dispersal processes can be overridden by pseudo‐turnover determined by signals inducing or breaking dormancy (e.g. due to changes in habitat quality) resulting in a low importance of habitat configuration and size. In this study, I investigated the turnover dynamics of 5 annual plant species growing on ant mounds of Lasius flavus over three years. I analyzed whether the grassland‐scale dynamics of these annuals is influenced by dispersal processes, or alternatively, by pseudo‐turnover of soil seed populations. For that purpose I 1) searched for populations formed from soil seeds only, 2) compared the relative contribution of the soil seed bank and seed rain for population restoration after disappearance from the vegetation and 3) investigated whether colonization and extinction events are affected by patch isolation. I assumed if population turnover was rather a result of the soil seed bank dynamics then spatial effects would be hard to detect. In spite of the presence of populations formed from soil seed and the relatively more important soil seed bank for potential population reestablishment, turnover dynamics followed the predictions of metapopulation theory. Population appearance was more probable in larger and less isolated patches. Probability of disappearance increased with decrease of population size that was negatively influenced by the patch size and its isolation. These findings indicate dispersal processes to be important in the turnover dynamics and only limited contribution of soil seed populations. Their small effectiveness is probably related to the low chance of recurrent disturbance on the mound surface.

Population genetics and conservation of the critically endangeredClematis acerifolia(Ranunculaceae)

Allozyme electrophoresis was used to evaluate the levels of genetic diversity and population genetic structure of the critically endangered Clematis acerifolia Maximowicz (Ranunculaceae), a narrow endemic species in China. On the basis of variation at 19 putative loci in nine populations covering the entire distribution of this species, low values of genetic diversity were detected (P = 20.5%, A = 1.27, and He= 0.072). A significant deficiency of heterozygotes was found in all populations. Most loci showed deviations from the Hardy–Weinberg equilibrium, probably as a result of population genetic structuring. The high genetic divergence among populations (FST= 0.273) can be interpreted as an effect of the extinction of local populations and genetic drift within extant populations, and has probably been enhanced by habitat fragmentation in recent decades. Threats to this species are mainly anthropogenic (road works, construction of holiday resorts, and extraction activities), although stochastic risks cannot be ignored. Therefore, to preserve extant genetic variation of C. acerifolia, in situ strategies, such as the preservation of its habitat or at least the most diverse populations, and ex situ measures, such as the collection and long-term storage of seeds, should be adopted.

A candidate locus for variation in dispersal rate in a butterfly metapopulation

Frequent extinctions of local populations in metapopulations create opportunities for migrant females to establish new populations. In a metapopulation of the Glanville fritillary butterfly (Melitaea cinxia), more mobile individuals are more likely to establish new populations, especially in habitat patches that are poorly connected to existing populations. Here we show that flight metabolic rate and the frequency of a specific allele of the metabolic enzyme phosphoglucose isomerase (pgi) were both highest in newly established, isolated populations. Furthermore, genotypes with this pgi allele had elevated flight metabolic rates. These results suggest that genetic variation in pgi or a closely linked locus has a direct effect on flight metabolism, dispersal rate, and thereby on metapopulation dynamics in this species. These results also contribute to an emerging understanding of the mechanisms by which population turnover in heterogeneous landscapes may maintain genetic and phenotypic variation across populations.

Sampling from Natural Populations with RNAi Reveals High Outcrossing and Population Structure in Caenorhabditis elegans

Despite a nearly worldwide distribution in nature, Caenorhabditis elegans exhibits low levels of genetic polymorphism, possibly as an indirect consequence of low levels of outcrossing. In the laboratory, Caenorhabditis elegans males are produced at low rates and are steadily eliminated from cultures, so that reproduction happens largely through self-fertilization in hermaphrodites. C. elegans is increasingly the focus of evolutionary research; however, natural outcrossing rates are difficult to measure because mating tests with laboratory strains are usually required to identify C. elegans. We sampled natural populations of C. elegans with an RNA interference (RNAi) assay. Heterozygosities and polymorphism patterns revealed surprisingly high levels of population structure and outcrossing (approximately 22% of individuals are estimated to be the result of outcrossing and not self-fertilization). The finding of strong local population structure, together with low levels of diversity on local and global scales, suggests a metapopulation model of frequent extinction and recolonization of local populations. The occurrence of substantial outcrossing suggests that the extinction of local populations is probably not driven by the accumulation of harmful mutations.

Spatiotemporal allozyme variation in the damselfly, Lestes viridis (Odonata: Zygoptera): gene flow among permanent and temporary ponds

Several insect species seem to persist not only in permanent but also in temporary ponds where they face particularly harsh conditions and frequent extinctions. Under such conditions, gene flow may prevent local adaptation to temporary ponds and may promote phenotypic plasticity, or maintain apparent population persistence. The few empirical studies on insects suggest the latter mechanism, but no studies so far quantified gene flow including both pond types. We investigated the effects of pond type and temporal variation on population genetic differentiation and gene flow in the damselfly Lestes viridis in northern Belgium. We report a survey of two allozyme loci (Gpi, Pgm) with polyacrylamide gel electrophoresis in 14 populations from permanent and temporary ponds, and compared these results with similar data from the same permanent populations one year before. The data suggested that neither pond-drying regime, nor temporal variation have a substantial effect on population genetic structuring and did not provide evidence for stable population differentiation in L. viridis in northern Belgium. Gene flow estimates were high within permanent and temporary ponds, and between pond types. Our data are consistent with a source-sink metapopulation system where temporary ponds act as sinks in dry years, and are quickly recolonized after local population extinction. This may create a pattern of apparent population persistence of this species in permanent and temporary ponds without clear local adaptation.

The control of emigration and its consequences for the survival of populations

Dispersal is the key process enhancing the long-term persistence of metapopulations in heterogeneous and dynamic landscapes. However, any individual emigrating from a occupied patch also increases the risk of local population extinction. The consequences of this increase for metapopulation persistence likely depend on the control of emigration. In this paper, we present results of individual-based simulations to compare the consequences of density-independent (DIE) and density-dependent (DDE) emigration on the extinction risk of local populations and a two-patch metapopulation. (1) For completely isolated patches extinction risk increases linearly with realised emigration rates in the DIE scenario. (2) For the DDE scenario extinction risk is nearly insensitive to emigration as longs as emigration probabilities remain below ≈0.2. Survival chances are up to half an order of magnitude larger than for populations with DIE. (3) For low dispersal mortality both modes of emigration increase survival of a metapopulation by ca. one order of magnitude. (4) For high dispersal mortality only DDE can improve the global survival chances of the metapopulation. (5) With DDE individuals are only removed from a population at high population density and the risk of extinction due to demographic stochasticity is thus much smaller compared to the DIE scenario. With density-dependent emigration prospects of metapopulations survival may thus be much higher compared to a system with density-independent emigration. Consequently, the knowledge about the factors driving emigration may significantly affect our conclusions concerning the conservation status of species.

Sustainable management of wildlife habitat and risk of extinction

Whether land management planning provides for sufficient habitat to sustain viable populations of indigenous wildlife is one of the greatest challenges confronting resource managers. Analyses of the effects of land management on natural resources often rely on qualitative assessments that focus on single species to reflect the risk of wildlife extinction across a planning area. We propose a conceptual framework for sustainable management of wildlife habitat that explicitly acknowledges the greater risk of an extinction event when considering the viability of multiple species, e.g., an indigenous vertebrate fauna. This concept is based on the principle that the likelihood of at least one event (i.e., species extinction) is the joint probability of the extinction probabilities of individual species, assuming independence among species’ responses to disturbance. We present an ecological rationale to support the view that, at a spatial scale of 10 4–10 6 ha (i.e., planning area) and a temporal scale of 10 2 years (i.e., planning horizon), wildlife species operating at varying ecological scales respond relatively independently to disturbances typically associated with land management. We use a hypothetical scenario of a wildlife viability assessment and Monte Carlo simulation to demonstrate that the probability of ‘any extinction’ is consistently higher than the probability of the ‘single most likely’ extinction, and that the difference between these values increases as more disturbance-sensitive species (i.e., species at risk) are analyzed. We conclude that risk assessments that rely upon the most sensitive single species may substantially underestimate the risk of wildlife extinction across a planning area. Furthermore, the selection of a planning alternative based on relative threat of local extinction of wildlife populations can vary depending on which paradigm is used to estimate risk to viability across the planning area.

Environmental variability in the early rearing environment generates behaviourally flexible cod: implications for rehabilitating wild populations

The release of hatchery-reared fishes for restoring threatened and endangered populations is one of the most controversial issues in applied ecology. A central issue has been to determine whether releases cause extinction of local wild populations. This may arise either through domesticated or non-local fishes hybridizing with wild fishes, or through inappropriate behavioural interactions; for example, many hatchery fishes show exaggerated aggressive and competitive behaviour and out-compete wild counterparts. The impact of the impoverished hatchery environment in shaping behaviour is only now receiving attention. Attempts to counteract hatchery-related behavioural deficiencies have utilized intensive training programmes shortly before the fishes are released. However, we show here that simple exposure to variable spatial and foraging cues in the standard hatchery environment generates fishes with enhanced behavioural traits that are probably associated with improved survival in the wild. It appears that fishes need to experience a varying and changeable environment to learn and develop flexible behaviour. Using variable hatchery rearing environments to generate suitable phenotypes in combination with a knowledge of appropriate local genotypes, rehabilitation of wild fishes is likely to succeed, where to date it has largely failed.

Defragmentation in the Netherlands: A Success Story?

In the Netherlands the problem of habitat fragmentation due to transport corridors is being addressed by a Long-Term Defragmentation Programme. Priority spots for defragmentation measures (i.e., locations where wildlife passages are most urgently required) were assessed by combining population viability modelling with local knowledge of critical road sections. Monitoring studies proved that wildlife passages are frequently used by a variety of wildlife species, but only if regularly inspected and maintained. Model simulations suggest that improperly managed passages may result in the extinction of local populations. More robust passages may considerably mitigate problems associated with poor management. In order to judge the success of the implemented defragmentation measures, their long-term effect on population viability needs to be assessed in future monitoring studies.

Current status of the threatened Dupont's lark Chersophilus duponti in Spain: overestimation, decline, and extinction of local populations

The European population of Dupont's lark Chersophilus duponti, restricted to Spanish steppe, was estimated to be 13,000 pairs in c. 50 populations in 1988. There is, however, recent evidence that this number was overestimated because of the previous use of line transects for estimating population sizes. In 2002–2004 we surveyed 34 previously known local populations in patches of variable size across half of its Spanish distribution. We found 13 (38%) local populations to be extinct, and a total of only 283–339 territories in 17 of the extant populations, seven of which held <5 territories. This census contrasts dramatically with the estimate of c. 3,000 pairs in the same populations in the 1980s. We estimate that the present Spanish population is c. 1,300 pairs. The discrepancy between this and earlier estimates is partly but not entirely due to previous overestimations. Habitat loss due to ploughing, and possibly habitat degradation and the resulting stochastic extinction of small and isolated populations, have contributed to the species' decline. Urgent research is needed to design and manage an adequate network of steppe patches that will ensure the long-term existence of this species. We recommend that Dupont's lark be categorized on the IUCN Red List as Endangered, both globally and nationally within Spain.

Genetic variation and biogeography of Mertensia maritima (Boraginaceae)

To elucidate the population dynamics and biogeographic connections of Mertensia maritima in the North Atlantic we studied the genetic variation within and among 16 local populations in the Oslofjord region (south‐east Norway), Finnmark (northern Norway), Iceland and Svalbard, using starch gel isozyme electrophoresis. We also tested the self‐compatibility experimentally. Mertensia maritima was found to be self‐compatible and able to self‐pollinate by autodeposition. The total level of genetic variation was extremely low. All the screened individuals were homozygous at all loci. All individuals from the populations of mainland Norway and Iceland were isozymatically identical, whereas individuals from Svalbard differed from these at three loci (AAT‐1, PGM‐3 and SKD‐2). The genetic identity between any population at Svalbard and any population on the mainland was 0.86. These genetic patterns may have resulted from a combination of inbreeding, frequent local population extinctions, and long‐distance dispersal. The isozymatic distinctness of the Svalbard populations is consistent with the arctic distribution of the morphologically based var. tenella, here raised to subspecific rank as Mertensia maritima ssp. tenella.

Was there a population replacement during the Late mid-Holocene in the southeastern Pampas of Argentina? Archaeological evidence and Paleoecological basis

Based on radiocarbon and craniofacial morphological evidence, we propose that a population replacement took place in the southeastern Pampas of Argentina sometime between 6000 and 3500 years BP. The analysis of the available radiocarbon database shows that there is a significant gap between 5960 and 5060 14C years BP. The morphometric analyses of samples of human crania corresponding to the early mid-Holocene (ca. 8000–6000 14C years BP) and to the early late-Holocene (ca. 3500–2000 14C years BP) strongly suggest that they probably belonged to at least two different biological populations. We contend that this population event must be considered the outcome of a local population contraction/extinction process, followed by the later expansion/dispersal of a new population into the area. This is consistent with the expectations of metapopulation ecology and evolutionary geography, which predict that increasing extinction risks select for higher dispersal rates, since local population contraction or extinction often result in empty or thinly populated patches, whose existence makes dispersal both feasible and profitable. A slightly corrected version of the model advanced by Politis (1984), that establishes a linkage between the climatic induced demographic and range contraction of critical mammalian resources (e.g., guanaco) and those of human populations, is still useful to explain situations such as the alleged mid-Holocene population replacement event.

SPIDER EFFECTS ON PLANTHOPPER MORTALITY, DISPERSAL, AND SPATIAL POPULATION DYNAMICS

Nonlethal (trait-mediated) effects of predators on prey populations, particularly with regard to prey dispersal, scarcely have been considered in spatial ecological studies. In this study, we report on the effects of spider predators on the mortality, dispersal, and spatial population dynamics of Prokelisia crocea planthoppers (Hemiptera: Delphacidae) in a prairie landscape. Based on a three-generation survey of host-plant patches (Spartina pectinata; Poaceae), the density of cursorial and web-building spiders declined significantly with increasing patch size (a pattern the opposite of that for the planthopper). Independent of patch size effects, an increase in the density of web-building and cursorial spiders had a negative effect on planthopper density in one of three generations each. Finally, the likelihood of extinction of local (patch) populations of planthoppers increased significantly with an increase in the density of web-building spiders. Planthoppers in small host-plant patches with high densit...

On optimal choices in increase of patch area and reduction of interpatch distance for metapopulation persistence

Metapopulation theory teaches that the viability of metapopulations may be enlarged by decreasing the probability of extinction of local populations, or by increasing the colonization probability of empty habitat patches. In a metapopulation model study it has recently been found that reducing the extinction probability of the least extinction-prone patch and increasing the colonization probability between the two least extinction-prone patches are the best options to prolong the expected lifetime of a metapopulation. In this article I examine with a more detailed model whether this translates into enlarging the largest patch and reducing the interpatch distance between the largest patches. Using two measures of metapopulation persistence (longevity and resilience) I found, firstly, that the largest patch should generally be enlarged if the choice is to enlarge a patch by a certain percentage (relative change) and that the smallest patch should generally be enlarged if the choice is to enlarge a patch by a fixed amount of area (absolute change), and secondly that indeed one should reduce the interpatch distance between the two largest patches, if the choice is among all pairs of patches. The strength of these rules of thumb (particularly the second part of the first rule) depends on the parameter values, particularly the amount of clustering of patches, the mean dispersal distance and the number of dispersers. Also, the rules of thumb are less pronounced when resilience is chosen as a measure of metapopulation persistence than when longevity is chosen.

Metapopulation structure of stream‐dwelling Dolly Varden charr inferred from patterns of occurrence in the Sorachi River basin, Hokkaido, Japan

Summary1. We investigated the occurrence (presence or absence) of stream‐dwelling Dolly Varden (Salmonidae: Salvelinus malma) in the 78 tributaries of the Sorachi River system, Hokkaido, Japan to examine evidence for metapopulation structure.2. We hypothesised (i) that the occurrence of Dolly Varden would be less likely in small tributaries because of the higher extinction risk and (ii) that occurrence would be less likely in isolated tributaries because of reduced rates of dispersal and colonisation. In addition to these two general predictions, we hypothesised (iii) that the occurrence would be more likely in spring‐fed tributaries, because Dolly Varden prefer cold water as present in these systems.3. When all the tributaries were included in a multiple logistic regression analysis, habitat size and habitat type (spring‐fed versus non‐spring‐fed tributaries) were selected for the best model, whereas isolation was not important. However, when tributaries in the lower Sorachi basin (an area likely influenced by human activities) were excluded from the analysis, the effect of isolation became significant.4. Our results indicate there is a potential metapopulation structure of the Dolly Varden in the Sorachi River basin. Furthermore, it is possible that metapopulation dynamics follow a clinal pattern. In the upper and middle reaches of the Sorachi River basin, an equilibrium between extinction and colonisation of Dolly Varden populations is a possibility. In the lower portion of the basin, isolation appeared to be less important, and extinction rate may be driving patterns of occurrence toward systematic extinction of local populations.5. Our results illustrate the possibility of spatial variation in the processes influencing occurrence of Dolly Varden within a single river basin, and the potential influences of habitat alteration on extinction and colonisation processes.